1. Field of the Invention
This invention relates to xerographic imagers utilizing a light emitter array. In particular, this invention is directed to architectures, characteristics and methods of using a time delay and integration (TDI) technique in organic light emitting diode (OLED) printbars used in such xerographic light emitter arrays.
2. Technical Background
One of the fundamental design challenges for xerographic imaging is getting enough light to the photoreceptor to enable sufficient print speed while providing adequate service lifetime of the printbar. Rapid progress in OLEDs has produced devices which emit light levels greater than computer monitors (300 cd/m.sup.2) and fluorescent tubes (3000 cd/M.sup.2) in both white and in colors collectively spanning the visible spectrum.
Lifetime studies of OLEDs indicate that diode lifetime is determined to first order by the total charge passed through the OLED. Thus, the OLEDs operate for short times at high brightness or for long times at low brightness. The lower end of the OLED brightness range is most stable, generally sustaining lifetimes of greater than 10,000 hours. The higher end of the OLED brightness range is less stable. For example, OLED devices operating at 1500 cd/m.sup.2 currently have sustainable lifetimes of only about 500 hours.
In a one-dimensional page-width array of such currently available OLEDs there is not enough brightness to print documents xerographically at a reasonable speed with reasonable reliability for commercial uses. Table 1 outlines the technical data for a xerographic printer using a single row OLED printbar having OLED emitters operating at 1500 cd/m.sup.2. The printbar is illuminating a photoreceptor requiring about 7.5 ergs/cm.sup.2. Thus, the print speed of the single row device is about 0.29 pages/min. Moderate print speeds are above five pages/min, and a more desirable print speed is about 30 pages per minute. The brightness deficit determined by this rough calculation is about 100 .times., especially when considering that the brightness and print speed of the single row page-width array of OLEDs leave no room for dead time. Inorganic diode based printbars for example, typically have a duty cycle well under 50% to minimize image blurring on the photoreceptor. Furthermore, the calculated print-speed is the speed before degradation, where the lifetime for the devices is the time to 50% output decay.
TABLE 1 __________________________________________________________________________ TECHNICAL DATA FOR A CONVENTIONAL SINGLE ROW OLED PRINTBAR __________________________________________________________________________ Light Emitter Inputs Outputs Average Wavelength 590 nm Surface Luminous 0.4712 1 m/cm 2 Flux Avg. Luminous 450 1 m/W Surface Radiance 0.0010 W/cm 2 Efficacy LED Brightness 1500 cd/m2 Surface Radiance 10472.0 ergs/sec.cm 2 LED Current Density 25 mA/cm2 Photoreceptor 103.778 ergs/sec.cm 2 Irradiance Display Voltage 20 Volts Pixel Size 0.0085 cm Number of Rows 1 Pixel Current 1.79 uA Array Fill Factor 88% Array Emitting Area 0.26 cm 2 Optical Inputs Array Width 0.08 mm Lens Transmittance 90% Array Emission 27.50 ergs/sec Lens Effective F# 4.765 Array Current 6.623643 mA Lens Efficiency 1.0% Array (Max) Power 0.13 Watts Photoreceptor Dose 7.5 erg/cm 2 Power Efficiency 0.2094% Page Property Inputs Page Dose 5758.05 ergs Document exc. time 0 sec Page Time 209.42 sec Fast scan 300 in-1 Line Time 82.12 msec resolution Slow scan 300 in-1 Print Speed 0.287 pages/min resolution Fast scan length 14 in Data Rate 0.051 MHz Slow scan length 8.5 in Fractional line 100% time __________________________________________________________________________
The brightness deficit is currently too large to compensate simply by running the diodes harder. For example, operating the OLEDs even briefly at 15000 cd/M.sup.2 would require such a high bias that the OLEDs would quickly become inoperative. Furthermore, doing so would only increase the print speed of the single row array to 3 pages/minute. In addition, the total lifetime print volume of the xerographic imager (&lt;9,000 pages) is insufficient.
Commonly assigned U.S. patent application Ser. No. 08/785,232 filed Jan. 17, 1997 to Fork et al., entitled "Active Matrix organic LED Display Device," the disclosure of which is incorporated herein by reference in its entirety, provides circuitry for operating an active matrix array of OLEDs using analog or digital memory. The freedom to place emissive layers on top of existing circuitry allows three-dimensional integration in the design of structures with a nearly 100% fill factor. This is advantageous over inorganic LEDs, which generally require epitaxial growth and therefore prohibit this type of three-dimensional integration. The layout of the pixel circuitry may be optimized depending on various priorities such as maximum fill factor, color processing, ease of manufacture, or ease of operation.
Commonly assigned U.S. patent application Ser. No. 08/785,280 filed Jan. 17, 1997 to Fork, entitled "Integrating Xerographic Light Emitter Array," the disclosure of which is incorporated herein by reference in its entirety, discloses one approach for using OLEDs operated at modest light levels to expose a photoreceptor drum or belt. This is accomplished by staging an array of emitters in the slow scan direction, and clocking the data through pixel driving shift-registers synchronized with the movement of photoreceptor past the array in the slow scan direction. Increased emitter lifetime and the ability to operate at lower light levels are achieved in proportion to the number of stages.
Commonly assigned U.S. patent application Ser. No. 08/785,231 filed Jan. 17, 1997 to Fork, entitled "Self Replacing OLED Printbar," the disclosure of which is incorporated herein by reference in its entirety, proposes another way to extend the lifetime of an OLED-based printbar. This is accomplished by creating a plurality of OLED printbars on a substrate, having all printbars share common optics and selecting a working printbar element in the event of a printbar element failure. Thus, printing is accomplished from a single row of emitters which operate at a high brightness and current. When one row burns out or decays to a level insufficient for printing, that row is deactivated, a new row is activated and printing continues.